I have to admit that I'm a bit conflicted about this particular scenario.
In my family, my mother would never have that. Then again, my mother would never let us order an appetizer just for ourselves, either. Even now, as adults, the familial understanding is that if appetizers are ordered, everybody shares. Of course, this is the norm in my family, where we believe nothing should ever be shared except food and bathroom space.
I think if the mom was trying to make a point to her daughter, and if the mom wasn't also offering up her daughter's entree, then all the OP or anyone else can do is sit back and participate or not based on their own conscience. I don't necessarily agree with the mom or the method, but not knowing the dynamic, I don't want to call judgment on her, either.
I don't think the mom was wrong per say, but I would not have felt comfortable taking the food, either.